The present invention relates to a method for treating metal containing waste water while employing a vessel containing the waste water in which at least one anode and one cathode are disposed, the cathode comprising electrically conductive elements and in which the waste water is subjected to electrolysis during which the cathode elements are being moved and to an apparatus for practicing the method. See Australian Patent No. 46,691, which discloses a similar process using electrically conductive particles as the cathode elements.
Waste waters in the sense of the present invention are understood to mean all metal containing solutions obtained during technical processing. Such solutions are, for example, waste waters from mines, final liquors, wash waters, trickling waters, rinse waters from tanneries, electroplating operations, the manufacture of printed circuits, film developing, etc. Electrolytic processes have been in use for a long time for the recovery of metals from such solutions having a relatively high metal content, so-called concentrates and semiconcentrates. In this way, it is possible to directly cathodically separate a major portion of the metal content of such solutions and to reduce the metal content of the solutions. When a certain reduced metal content is reached, however, the current efficiency drops considerably, and further reduction in the metal concentration of the solutions is practically unattainable. The then remaining solutions cannot be economically processed with the aid of ion exchangers, since the requirement for regenerating chemicals for the ion exchanger would be unduly high and the resulting heavy salting of the waste water would constitute an additional stress on the environment.
In order to further reduce the residual metal content in the solution, it has often been proposed to improve the cathodic deposition conditions. Such proposals are essentially directed to reducing the depletion of the electrolyte of depositable metal ions in the region of the cathode. Thus, various types of electrolytic cells are known which contain spatially fixed, firmly contacted, quasi two-dimensional electrodes which are frequently arranged vertically, but which can also have another orientation. A relative movement of the electrodes with respect to the electrolyte serves to improve deposition conditions.
Movement of the electrodes can be realized by vibration. The electrodes may be designed to have a disc, ring or cylindrical shape, and may be rotated in the electrolyte. Further, the exterior configuration of the electrodes may contribute to the fact that the electrolyte flows against the electrode surface at high speed and, if possible, not merely in a laminar manner. The electrolyte may also be pumped through channel type electrolysis cells, and may flow vertically through concentrically arranged electrodes with gases being blown in simultaneously. The vertical movement of the liquid may be so intensive that glass beads or other particles of various materials are stirred up and improve the mass transfer at the vertical electrodes.
In order to realize a higher space-time yield during electrolytic processes, numerous efforts have been directed at building up three-dimensional electrodes from electrically conductive bulk material. Such particle piles are called fixed beds if the relative movement with respect to the electrolyte is effected by passing the electrolyte through the bed, either in the direction of the electric field lines or normally thereto.
Australian Patent No. 46,691 discloses a process as described above. At least one stirrer is provided to mechanically move a cathode composed of conductive particles disposed at the bottom of the vessel through which the electrolyte flows. In this process, the anode may also be constituted by electrically conductive particles and is disposed above the cathode pile. Nevertheless the cathode pile has dead spaces and potential free zones in which the electrolytically deposited metal is chemically redissolved in part. A further drawback of this process is the substantial amount of energy required and the resulting intensive heating of the electrolyte. In order to prevent too great a rise in temperature, the electrolyte must therefore be cooled at the cost of additional energy.